Evaluating hydrologic reservoir constraint in coal seams and shale formations

ABSTRACT

Methods and apparatus suitable for quickly and accurately measuring  13 C levels and supporting data in an aqueous fluid reservoir. Interpreting the resulting data to indicate key factors regarding a reservoir and completion methods, including reservoir constraint, gas producibility, and completion success. A sensor and to a sensing method that evaluates the level of hydrologic constraint in aquifers occurring in unconventional reservoirs, such as shales and coals is disclosed. Specifically, Raman spectroscopy is disclosed as a sensor and a sensing method that measures the level of naturally-occurring  13 C in an aqueous reservoir and compares the level of  13 C to the levels typical for highly constrained and highly unconstrained reservoirs. The disclosed sensor and sensing method also monitors the level of naturally-occurring  13 C in a reservoir. Also disclosed is a method of using δ 13 C Dic  to evaluate geographic areas of coal bed reservoir water having biologic methanogenic activity.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of U.S. application Ser. No.14/046,953, filed on Oct. 5, 2013, which claims the benefit ofPCT/US12/32300, filed on Apr. 5, 2012, which claims priority from61/472,623, filed on Apr. 7, 2011.

TECHNICAL FIELD

This disclosure provides a sensing method that evaluates levels ofhydrologic constraint in aquifers occurring in unconventionalreservoirs, such as shale formations and coal formations. Specifically,the disclosed sensing method measures levels of naturally-occurring ¹³Cin a reservoir and compares the level of ¹³C to the levels typical forhighly constrained and highly unconstrained reservoirs. The disclosedsensing method also monitors levels of naturally-occurring ¹³C in areservoir while operations that may unintentionally or intentionallychange the constraint in a reservoir are undertaken. Such operations caninclude, for example, cementing of casing in a wellbore intersectingmore than one reservoir, hydraulic fracturing of a reservoir, andhorizontal drilling in a reservoir.

BACKGROUND

Isotopic analysis of reservoir fluids in coal seams is a method toevaluate the type of chemical process producing coal bed methane gasretrieved from those seams (Dudley D. Rice, George E. Claypool, AAPGBulletin, Volume 65, Issue 1. (January), Pages 5-25 (1981)). This methodis possible because methanogenic consortia that digest the coal toproduce methane preferentially digest moieties containing ¹²C atoms.Thus, gases produced by methanogens are enriched in ¹²C.

The preferential metabolysis of ¹²C from coals or shales duringbiogenesis results in enrichment of ¹³C in the carbon containing saltsand other materials residual to the process. This enrichment has beenused to typecast fluids from particular coal seams (Enhancing MicrobialGas From Unconventional Reservoirs: Geochemical And MicrobiologicalCharacterization Of Methane-Rich Fractured Black Shales, FINAL REPORT(March 2004-September 2004) Prepared by: Anna M. Martini, KlausNüsslein, Steven T. Petsch, Prepared for: RESEARCH PARTNERSHIP TO SECUREENERGY FOR AMERICA, Subcontract No. R-520) and even to track theeventual impact of the fluids on the environment(http://faculty.gg.uwyo.edu/cfrost/pdfs/2010%20Frost%20CBNG%20book.pdf).But the enrichment of ¹³C in the carbon containing salts and othermaterials residual to the preferential metabolysis of ¹²C from coals orshales during biogenesis has not been used in the manner of the presentdisclosure, that is, to measure hydrologic constraint. Thesemeasurements are made while operations may be undertaken thatunintentionally or intentionally change the constraint in a reservoir.More specifically, “hydrologic constraint” refers to the extent to whichthe fluids resident in a coal seam or shale formation are residual fromthe methanogenic process. Thus, highly constrained reservoirs havelittle or no influx of foreign fluids that are not residual from themethanogenic process. Highly unconstrained reservoirs have appreciable,and sometimes significant, influx of foreign fluids that are notresidual from the methanogenic process. Such foreign fluids can include,but are not limited to, fluids from other geological formations, fluidsfrom surface waters, fluids from recharge at formation outcrop, andothers. Influx of such fluids can occur by a variety of mechanismsincluding, but not limited to, via a wellbore connecting multipleformations, via a multi-formation geological fault, and via fracturenetworks connected to similar cross formation flow pathways.

Coal is the result of coalification, which pertains to the degree ofbiogenic and then thermogenic transformation of organic sediments.Coalification occurs initially via methanogens. Methanogens performmethanogenesis, a type of biogenic process, which results in enrichmentof ¹³C isotope because ¹²C is consumed as it is easier to metabolize.Thermogenesis however, does not affect ¹³C levels to a significantextent. Moreover, surface water generally has non-enriched ¹³C levels.Gases produced by thermogenic processes, on the other hand, undergo nosuch kinetic preferential metabolysis and therefore are notsignificantly enriched in ¹²C. As a result, measurement of ¹²C levels inmethane gas produced from coals can indicate whether the coal isundergoing biogenic or thermogenic coalification.

The natural abundance of ¹²C isotopes is about 98.9% of carbon atoms.The natural abundance of ¹³C isotope is about 1.1%. In water or anaqueous media, the dissolved inorganic carbon (DIC) consists of carbondioxide/carbonic acids [CO₂+H₂CO₃], bicarbonate [HCO₃] and carbonate[CO₃ ²⁻]. Therefore, the tracer of interest is δ¹³C_(DIC), which isdefined as the ¹³C/¹²C ratio in the sample divided by the standard¹³C/¹²C ratio, minus one, and typically expressed in parts per thousandby then multiplying the result by one thousand. In coal beds, bacteriaferment acetate (CH₃COOH→CH₄+CO₂) and reduce carbon dioxide(CO₂+4H₂→CH₄+2H₂O). As a result, bacterial methanogenesis preferentiallyremoves ¹²C and the remaining DIC is enriched in ¹³C.

A positive δ¹³C_(DIC) identifies groundwater in which biogenicproduction of methane has occurred. The δ¹³C_(DIC) of groundwater thatis not associated with methanogenesis have ratios of negative 5% tonegative 20%, whereas water coproduced with coal bed methane has ratiosas high as +35%.

Therefore, there is a need in the art for methods using δ¹³C enrichmentmeasurements in coal and shale fluids to indicate practical reservoircharacteristics of commercial importance, such as hydrologic constraintin aquifers. There is also a need for sensor devices that can performsuch measurements rapidly and effectively in coal bed and shale fluidreservoirs. The present disclosure was made to address this need.

SUMMARY

The present disclosure provides a method for determining hydrologicconstraints in unconventional reservoirs, comprising:

(a) determining levels of naturally-occurring ¹³C in a typicalunconventional reservoir;

(b) determining the level of ¹³C in a target unconventional reservoir;and

(c) comparing the ¹³C levels between a typical unconventional reservoirto a target unconventional reservoir to show if there have been changesto hydrologic constraint from one or a plurality of operations in thetarget unconventional reservoir. For purposes of this disclosure, atypical unconventional coalbed methane reservoir may have ratios ofδ¹³C_(DIC) as high as +35%. A target unconventional reservoir is acoalbed or shale reservoir under investigation using the methods andapparatus of this disclosure.

Preferably, the unconventional reservoirs are coal bed reservoirs andshale fluid reservoirs. Preferably, the operations in the targetunconventional reservoir are selected from the group consisting ofcementing of casing in a wellbore intersecting more than one reservoir,hydraulic fracturing of a reservoir, horizontal drilling in a reservoir,and combinations thereof.

The present disclosure further provides a method for determininghydrologic constraints in unconventional reservoirs, comprising:

(a) determining levels of naturally-occurring ¹³C in a typicalUnconventional reservoir;

(b) determining the level of naturally-occurring ¹³C in a targetunconventional reservoir; and

(c) comparing the levels of ¹³C between the typical unconventionalreservoir and the target unconventional reservoir to determine whetherthe target unconventional reservoir has less reservoir constraints thanthe typical unconventional reservoir.

Preferably, the unconventional reservoirs are coal bed reservoirs orshale fluid reservoirs. Preferably, a difference in respective levels of¹³C between the typical and the target reservoirs is used to predictfuture production success or failure of the target reservoir.Preferably, the levels of ¹³C are determined by use of Ramanspectroscopy. Preferably, the operations in the target unconventionalreservoir are selected from the group consisting of cementing of casingin a wellbore intersecting more than one reservoir, hydraulic fracturingof a reservoir, horizontal drilling in a reservoir, and combinationsthereof. More preferably, the levels of ¹³C are compared to the levelsof levels of ¹²C in order to produce a δ¹³C enrichment level.

The present disclosure further provides a method for determininghydrologic constraints in unconventional reservoirs, comprising:

(a) determining or obtaining levels of naturally-occurring ¹³C in atypical unconventional reservoir;

(b) determining the level of ¹³C in a target unconventional reservoir aplurality of times; and

(c) comparing the ¹³C levels between the typical unconventionalreservoir to the target unconventional reservoir each time to show ifthere have been changes to hydrologic constraint from one or a pluralityoperations in the target unconventional reservoir.

The present disclosure also provides a method for determining extent ofhydraulic fracturing of a target unconventional reservoir, comprising:

(a) determining levels of naturally-occurring ¹³C in a targetunconventional reservoir;

(b) hydraulically fracturing the target unconventional reservoir tocreate a fractured reservoir;

(c) determining levels of ¹³C in the fractured reservoir; and

(d) comparing the levels of ¹³C in the target unconventional reservoirof step (a) to the fractured reservoir of step (c) to evaluate determineif non-reservoir fluids have begun entering the reservoir as a measureof extent of hydraulic fracturing.

Preferably, the unconventional reservoirs are coal bed reservoirs orshale fluid reservoirs. Preferably, a difference in respective levels of¹³C in the target reservoir before and after fracturing is used topredict future production success or failure of the target reservoir.Preferably, the levels of ¹³C are determined by use of Ramanspectroscopy. Preferably, the levels of ¹³C are compared to the levelsof levels of ¹²C in order to produce a δ¹³C enrichment level.

BRIEF DESCRIPTION OF THE FIGURES

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate preferred embodiments of theinvention. These drawings, together with the general description of theinvention given above and the detailed description of the preferredembodiments given below, serve to explain the principles of theinvention.

FIG. 1 shows representative Raman spectra of the bicarbonate ioncontaining (A) ¹²C and (B) ¹³C atoms dissolved in water.

FIG. 2 shows a representative log of δ¹³C enrichment in naturallyoccurring bicarbonate ions dissolved in reservoir fluid, as measureddownhole using a Raman spectrometer in a coalbed methane well.

FIG. 3 shows a representative graph of δ¹³C enrichment for reservoirfluids produced from a coal seam before and after hydraulic fracturingwas completed on the coal seam. The decrease in δ¹³C enrichment afterhydraulic fracturing indicated that the induced fractures connected toone or more adjoining fluid reservoirs, thereby reducing the coal seam'shydraulic constraint, increasing the fluid volumes necessary to beproduced in order to produce gas from the coal seam, and injectinghydraulic fracturing fluids and chemicals into the adjoining fluidreservoir or reservoirs.

FIG. 4 graphs positive δ¹³C_(DIC) measurement in produced waters fromvarious coalbed reservoirs, showing particularly highly enriched ¹³Cquantities in fields A and B of 4 fields (A through D).

FIG. 5 graphs gas production from 4 fields over 2.5 years.

FIG. 6 graphs the water gas ratio of 4 fields (A through D) as afunction of δ¹³C_(DIC).

DETAILED DESCRIPTION

Without being bound by theory, the hypothesis underpinning the disclosedmethod is that coals contain ¹³C enriched fluids. The level of δ¹³Cenrichment is a characteristic of each zone and field. Moreover, surfaceand non-coal aquifers do not have ¹³C enriched aqueous fluids.Therefore, measuring ¹³C levels confirms if an aqueous fluid comes froma coal bed. Further, any measured decrease in δ¹³C enrichment from abaseline measurement is thus directly proportional to an influx ofaqueous fluid from non-coal zones. As a result, the disclosed method isnovel and has not been employed during commercial exploration andproduction of coal bed methane and shale gas.

The present disclosure provides a new reservoir evaluation technologythat allows operators to high grade prospective shale and coalbed gasproperties and target pay zones, allowing operators to avoidnon-economic completion costs and water use. By providing a method andapparatus that can be used to quickly, accurately and inexpensivelyassess shale and coalbed gas resources at a high data density, thisdisclosure enables more complete evaluation of shale and coalbed gasresources. This evaluation focuses operators on developing completionmethods for shale and coalbed gas targets that warrant investment,reduce overall water usage, and increase the conversion of shale andcoalbed gas resources into shale and coalbed gas reserves. The disclosedmethod removes uncertainty regarding possible fluid influx intoreservoirs via recharge or invasion, incenting more rapid capitalinvestment by shale and coalbed gas operators and investors, andaccelerating natural gas production from these reservoirs. The disclosedmethod enables shale and coalbed gas operators to increase explorationsuccess rates, reduce finding costs, reduce completion costs, and reduceenvironmental impacts without constraining production.

Methods for Determining ¹³C Levels

Typical methods for measuring the relative levels of ¹²C and ¹³C influids produced from coals and shales involve the use of various type ofmass spectrometer laboratory instruments. In order to employ suchmethods, it is necessary to

-   -   1. collect fluid samples from a zone of interest, or from wells        completed in a zone of interest and producing fluid from that        zone;    -   2. insure that such samples are not affected by subsequent        oxidation or other spurious chemical processes or contaminants;    -   3. transport such samples to a laboratory equipped with        appropriate analytical equipment;    -   4. analyze the samples for ¹²C and ¹³C levels; and    -   5. compare those levels to the levels typical or expected for        the zone of interest in order to ascertain hydrological        constraint in the zone of interest.        This ex situ process is tedious, time consuming, costly and        limited in nature. Analysis of multiple fluid samples is        difficult and therefore evaluation of how fluid properties may        vary over time is not facilitated by this method. Therefore, a        need exists for a method for measuring the relative levels of        ¹²C and ¹³C in fluids produced from coals and shales that        operates in situ.

Disclosed herein is a method of using an optical spectrometer to measurein situ the relative levels of ¹²C and ¹³C in fluids produced from coalsand shales. This method involves measuring the fundamental vibrationalfrequencies for molecular bonds and moieties in carbon containing salts.Such vibrational frequencies depend on the weight of the atomsparticipating in the vibrational moments. Thus, the fundamentalvibrational frequencies of ¹²C containing molecular bonds occur athigher energies than the fundamental vibrational frequencies of ¹³Ccontaining molecular bonds and moieties.

A variety of optical spectroscopies, including but not limited to Ramanspectroscopy, infrared spectroscopy, and near-infrared spectroscopy,measure the fundamental vibrational frequencies, or combinations offundamental vibrational frequencies, in molecules.

In Raman spectroscopy, the shift in vibrational frequency between ¹²Cand ¹³C is approximately proportional to the vibrational shift expectedfor a spring connecting two masses, where one mass is increased in massfrom 12 mass units to 13 mass units. For bicarbonate, the reduction invibrational frequency for a molecule containing ¹³C (as compared to amolecule containing ¹²C) is about 1.8% or about 20 wavenumbers.Representative spectra of both ¹²C and ¹³C containing bicarbonate iondissolved in water are shown in FIG. 1.

Therefore, a Raman spectrometer capable of resolving the ¹²C and ¹³Cpeaks, and detecting accurately changes in the relative sizes of thosepeaks, can be used to measure the relative δ¹³C enrichment in coal seamand shale gas reservoir fluids.

Raman spectroscopy has been successfully adapted for use in coal seamand shale gas wells, as set forth in previously incorporated U.S. Pat.No. 6,678,050 to Pope et al. Therefore, this invention discloses for thefirst time the use of Raman spectrometers to measure ¹³C enrichmentlevels in situ, downhole, in coal seam and shale gas wells.

Likewise, Raman spectroscopy has been successfully adapted for use inanalyzing at the wellhead fluids produced from coal seam and shale gasreservoirs. See pending application entitled In-Situ Detection AndAnalysis Of Methane In Coal Bed Methane Formations With Spectrometers,Ser. No. 61/602,939, filed Feb. 24, 2012 which is incorporated in itsentirety by reference herein. Therefore, this invention discloses forthe first time the use of Raman spectrometers to measure δ¹³C enrichmentlevels in situ, at the wellhead, in coal seam and shale gas fluiddischarge streams.

In a preferred embodiment, a Raman spectrometer suitable for use in awell is obtained using previously disclosed inventions. One such Ramanspectrometer is the device used by the assignee, which is anepi-illuminated, grating equipped Raman spectrometer with a 532 nm laserexcitation and charge-coupled device (CCD) detector packaged into a3-inch diameter, 80 inch long high pressure steel housing. Thespectrometer interrogates the surrounding gases or fluids via a one-inchsapphire window located at bottom of its housing. The spectrometercommunicates to a surface computer and power unit via a standardfour-conductor wireline.

The Raman spectrometer wireline tool top sub is connected to a cablehead that is connected to a four-conductor wireline spool of appropriatelength. The spectrometer is powered up by connecting a power supply totwo of the four wires in the wireline. The surface computer communicateswith and controls the spectrometer via the other two wires.

The spectrometer is operated at the wellhead in order to check itsoperation and calibration. It is then lowered into the well while Ramanspectra are collected at 10 second intervals and then transmitted to thesurface computer over a subsequent 20 seconds. When the spectrometerencounters reservoir fluid in the wellbore, the Raman spectrumcharacteristic of that fluid is measured and transmitted to the surfacecomputer. The human operator reviews that spectrum for quality andcharacter to confirm its origin, and the computer processes thatspectrum for spectral features of interest, such as position of peaksand number of photons comprising one or more particular peaks.

In a well containing ¹²C and ¹³C isotopes of particular chemicals, suchas dissolved bicarbonate, the Raman spectrometer measures one positionand number of photons comprising a ¹²C-related bicarbonate vibrationalmode and a different position and number of photons comprising a peakcorresponding to the ¹³C-related bicarbonate vibrational mode. The humanoperator and/or the computer then evaluate the positions and number ofphotons for each peak and relate them to each other and/or to acalibration scale measured under controlled laboratory conditions. Inthis manner, the amount of δ¹³C enrichment in a reservoir fluid isquickly and conveniently measured in a wellbore.

For example, a five inch diameter steel casing was completed into atwelve-foot thick coal seam by perforation of the casing. The coal seamwas located at 890 feet below surface to 902 feet below surface.Perforations were made from 892 feet below surface to 900 feet belowsurface. The perforations were performed with 50 feet of water cushionabove the perforating gun. The coal seam subsequently flowed 340 feet ofadditional fluid into the wellbore, so that the wellbore contained 390feet of fluid total above the mean depth of the coal seam. The top ofthe fluid column was therefore located at 506 feet below surface. Athree-inch diameter, 80 inch long wireline Raman spectrometer tool wassubsequently lowered into the wellbore while it was continuouslycollecting Raman spectra. When it encountered fluid in the wellbore, theRaman spectrometer subsequently collected and transmitted Raman spectraof that fluid (primarily water) and of the analytes in that fluid(primarily methane and bicarbonate containing both ¹²C and ¹³Cisotopes). The spectra were then processed by the computer according toalgorithms well known to those skilled in the art of vibrationalspectroscopy to produce the concentrations of each bicarbonate isotopeat each measured depth. Those concentrations were then mathematicallycompared to produce a 30% average δ¹³C enrichment in the dissolvedbicarbonate present at each depth in the fluid column, as shown in FIG.2. The self consistent character of the resulting data at 556 feet belowsurface reflects the fact that the wellbore contained reservoir fluid atdepths greater than 556 feet below surface. This fact was corroboratedby examination of the methane concentration and total dissolved solidsmeasured concurrently throughout the fluid column. Based on this result,the coal seam in which that well was completed was judged to be highlyconstrained. Since the methane gas content measured for the coal seamwas of economic quantities, and since the drawdown required to producegas from the coal seam was measured to be only 145 psia, the coal seamwas judged to be an attractive production target.

Similar measurements can be undertaken using a variety of fluid handlingtechniques. For example, the example analysis described above could alsobe undertaken for multiple coal seams intersected by a cased andcemented wellbore where the casing is perforated sequentially startingwith the deepest coal and fluid from each zone is isolated in the casingusing an bridge plug, inflation packer, or similar device and Ramanspectroscopy testing of δ¹³C enrichment is subsequently undertaken oneach isolated fluid.

In a further example, the example analysis described above could also beundertaken for multiple coal seams intersected by a cased and cementedwellbore where the casing is perforated in all zones and then the fluidfrom each zone is isolated and produced using a drill stem testingstring, straddle packer, or similar device and Raman spectroscopytesting of δ¹³C enrichment is subsequently undertaken on each isolatedfluid.

In a further example, the example analysis described above could also beundertaken for multiple coal seams intersected by an uncased wellborewhere each seam is isolated and fluid from each seam is produced using adrill stem testing string, straddle packer, or similar device and Ramanspectroscopy testing of δ¹³C enrichment is subsequently undertaken oneach isolated fluid.

In a further example, the example analysis described above could also beundertaken wherein the Raman spectroscopy testing of δ¹³C enrichment isperformed using a Raman spectrometer located at the wellhead that isconnected to a fiber optic assembly that is subsequently lowered intothe wellbore to contact the fluid. The Raman spectroscopy analysis isthen undertaken using the fiber optic assembly to deliver the excitationradiation and collect the scattered photons.

In some cases, it may be useful to compare the δ¹³C enrichmentmeasurements described herein with standard water, gas and coal analysisresults in order to describe more comprehensively the coal seam or shalegas reservoir. Such comparison can assist in interpretation of the datasets. The combination of such analyses with the δ¹³C enrichmentmeasurements described herein is considered a part of the inventiondisclosed here.

Methods of Using Determined ¹³C Levels

This invention discloses for the first time the use of δ¹³C enrichmentdata to assess whether drilling and completion operations create newflow channels for fluids between the target coal or shale zone and otherzones near a wellbore. In particular, this invention describes a methodof testing an untreated coal or shale zone in order to establish itsnaturally characteristic δ¹³C enrichment level, treating the coal orshale zone, retesting the coal or shale zone to evaluate whether thetreatment created new fluid flow channels to other zones, and then usingthat information to determine whether the coal or shale zone is anappropriate production target. That information can also be used todetermine whether the treatment resulted in contamination to thesurrounding zones. That information can also be used to refine thetreatment itself in order to reduce or eliminate said contamination.

For example, a five inch diameter steel casing was completed into atwelve-foot thick coal seam by perforation of the casing. The coal seamwas located at 890 feet below surface to 902 feet below surface.Perforations were made from 892 feet below surface to 900 feet belowsurface. The perforations were performed with 50 feet of water cushionabove the perforating gun. The coal seam subsequently flowed 340 feet ofadditional fluid into the wellbore, so that the wellbore contained 390feet of fluid total above the mean depth of the coal seam. The top ofthe fluid column was therefore located at 506 feet below surface. Athree-inch diameter, 80 inch long wireline Raman spectrometer tool wassubsequently lowered into the wellbore while it was continuouslycollecting Raman spectra. When it encountered fluid in the wellbore, theRaman spectrometer subsequently collected and transmitted Raman spectraof that fluid (primarily water) and of the analytes in that fluid(primarily methane and bicarbonate containing both ¹²C and ¹³Cisotopes). The spectra were then processed by the computer according toalgorithms well known to those skilled in the art of vibrationalspectroscopy to produce the concentrations of each bicarbonate isotopeat each measured depth. Those concentrations were then mathematicallycompared to produce a 30% average δ¹³C enrichment in the dissolvedbicarbonate present at each depth in the fluid column, as shown in FIG.2. The self consistent character of the resulting data at 556 feet belowsurface reflects the fact that the wellbore contained reservoir fluid atdepths greater than 556 feet below surface. This fact was corroboratedby examination of the methane concentration and total dissolved solidsmeasured concurrently throughout the fluid column. Based on this result,the coal seam in which that well was completed was judged to be highlyconstrained. Since the methane gas content measured for the coal seamwas of economic quantities, and since the drawdown required to producegas from the coal seam was measured to be only 145 psia, the coal seamwas judged to be an attractive production target. Hydraulic fracturingof the coal seam was undertaken in order to increase its effectivepermeability in order to increase its water and gas flow rates. 30,000barrels of fluid were injected into the seam at 1,500 psia in order tofracture the seam. Subsequently, fluid was produced back until thesolution gas and salinity measured for the fluid became constant,indicating that the frac fluids had all been removed from the formationand the produced fluid was reservoir fluid. Subsequently, fluid wassampled from the water discharge pipe and conveyed to a laboratory whereδ¹³C_(DIC) was measured to be 12% enriched. FIG. 3 shows a graph of theδ¹³C_(DIC) in fluids collected from the coal seam before and afterhydraulic fracturing. Because the post-frac fluid δ¹³C_(DIC) was reducedsubstantially from the pre-frac δ¹³C_(DIC) level, it was concluded thatthe hydraulic fracturing process fractured through the coal seam andinto an overlaying limestone.

One group of Atlantic Rim springs have positive δ¹³C_(DIC). EnrichedLewis shale springs have Na—HCO₃-type water, and some of them emitmethane. Enriched Lewis shale springs have higher total dissolvedcontents (TDS) of 800-4000 mg/L than springs with negative δ¹³C_(DIC)(TDS typically 1064<1000 mg/L). These springs occur together in clustersup dip of the coal bed natural gas (CBNG) production areas. Thevariations in abundance of methane emerging from these springs may berelated to local variations in hydrostatic pressure of the near-surfacemethane-bearing reservoirs or influx from other gas-saturated coal bedreservoirs. Although water produced with CBNG are Na—HCO₃-type waterwith relatively high TDS (>1000 mg/L) and positive δ¹³C_(DIC). Watersamples from wells with low water/gas ratios have the highest δ¹³C_(DIC)and produce the most gas. Water samples from wells with lower, althoughstill positive, δ¹³C_(DIC) values reflect the addition of isotopicallylight water from other reservoirs. Structure contour mapping identifiesfaults in proximity to low δ¹³C_(DIC) wells. Geochemical analysis ofδ¹³C_(DIC) help to identify CBNG and shale natural gas reservoirs withthe highest potential for natural gas production while minimizingunnecessary water production.

The present disclosure predicts a correlation between water/gas ratios,production potential, efficiency, and δ¹³C_(DIC). δ¹³C_(DIC) should beless enriched in coal bed reservoirs that are hydraulically connected tomultiple water sources with isotopically light carbon compositions. Anunconfined coal bed reservoir is likely to produce more water duringCBNG production than a confined coal bed reservoir, as the hydraulicpressure of multiple connected reservoirs would need to be reduced topromote CBNG desorption. A group of wells in a single coal bed, whichhave highly enriched δ¹³C_(DIC) values and low cumulative water/gasratios, appear to be producing from confined coal bed reservoirs.Another group of wells, which are not as enriched regarding δ¹³C_(DIC)and have high cumulative water/gas ratios, are likely producing frompartially confined or unconfined coal bed reservoirs. δ¹³C_(DIC) mayhelp producers determine in which geographic areas coal bed reservoirwater has undergone the most biologic methanogenic activity and/or thosecoal bed reservoirs that are confined.

Enrichment of δ¹³C_(DIC) is directly related to bacterialmethanogenesis. The generation of thermogenic gas will not enrich DICratios, and δ¹³C_(DIC) characterization is inapplicable in CBNGreservoirs that are exclusively thermogenic. The δ¹³C_(DIC) alone cannotidentify which specific isotopically light reservoirs are incommunication with coal bed reservoirs. Water quality, well log, andgeologic interpretation of likely reservoirs may aid in distinguishingthese reservoirs.

A δ¹³C_(DIC) measurement can characterize CBNG reservoirs. Moreover,carbon isotopic tracers in produced water can guide CBNG development tothose areas where natural gas production is maximized and waterproduction is minimized. Positive δ¹³C_(DIC) of CBNG produced waterindicates that bacterial methanogenesis is associated with these waters(FIG. 4). However, a significant range in δ¹³C_(DIC) exists in Field ACBNG samples, which provides additional information about the potentialfor CBNG production from individual wells and related reservoirs (FIG.5).

Analyte concentrations also increase with distance from faults. Thesemeasurements can determine if CBNG reservoirs and shale bed natural gas(SBNG) reservoirs in a particular area are partially confined orunconfined as a result of faulting. This is because water withisotopically light carbon compositions and lesser amounts of brine tomix into the coal bed reservoir.

δ¹³C_(DIC) measurements can analyze CBNG production potential,particularly in hydraulically confined CBNG reservoirs. This capabilityalone makes δ¹³C_(DIC) a significant coal bed reservoir and CBNGanalysis tool (FIG. 6). A correlation is predicted between water/gasratios, production potential, efficiency, and δ¹³C_(DIC), as: (a)δ¹³C_(DIC) should be less enriched in coal bed reservoirs that arehydraulically connected to multiple water sources with isotopicallylight carbon compositions. (b) An unconfined coal bed reservoir islikely to produce more water during CBNG production than a confined coalbed reservoir, as the hydraulic pressure of multiple connectedreservoirs would need to be reduced to promote CBNG desorption.

Enrichment of δ¹³C_(DIC) is directly related to bacterialmethanogenesis. The generation of thermogenic gas will not enrich DICratios, and δ¹³C_(DIC) characterization is inapplicable in CBNGreservoirs that are exclusively thermogenic. δ¹³C_(DIC) alone cannotidentify which specific isotopically light reservoirs are incommunication with coal bed reservoirs. Water quality, strontiumisotopic compositions, well log, and geologic interpretation of likelyreservoirs may aid in distinguishing these reservoirs. δ¹³C_(DIC)analysis records the characteristics of the produced water at the timethe wells were sampled.

The foregoing disclosure has been set forth merely to illustrate theinvention and is not intended to be limiting. Since modifications of thedisclosed embodiments incorporating the spirit and substance of theinvention may occur to persons skilled in the art, the invention shouldbe construed to include everything within the scope of the appendedclaims and equivalents thereof.

This specification is to be construed as illustrative only and is forthe purpose of teaching those skilled in the art the manner of carryingout the invention. It is to be understood that the forms of theinvention herein shown and described are to be taken as the presentlypreferred embodiments. As already stated, various changes may be made inthe shape, size and arrangement of components or adjustments made in thesteps of the method without departing from the scope of this invention.For example, equivalent elements may be substituted for thoseillustrated and described herein and certain features of the inventionmaybe utilized independently of the use of other features, all as wouldbe apparent to one skilled in the art after having the benefit of thisdescription of the invention.

While specific embodiments have been illustrated and described, numerousmodifications are possible without departing from the spirit of theinvention, and the scope of protection is only limited by the scope ofthe accompanying claims.

We claim:
 1. A method for determining extent of hydraulic fracturing ofa target reservoir comprising: determining levels of ¹²C and ¹³Cisotopes in reservoir fluids present in the target reservoir usingspectroscopy; hydraulically fracturing the target reservoir to create afractured target reservoir; determining levels of ¹²C and ¹³C isotopesin reservoir fluids present in the fractured target reservoir usingspectroscopy by lowering a Raman spectrometer until reservoir fluid isencountered and then sampling the reservoir fluid, wherein thespectroscopy of the levels of ¹²C and ¹³C isotopes is performed on thereservoir fluid and the analytes in the reservoir fluid in the targetreservoir, wherein the analytes comprise bicarbonate or a combination ofmethane and bicarbonate; comparing levels of ¹²C and ¹³C isotopes in thefluids of the target reservoir to the fluids of the fractured targetreservoir to determine if non-reservoir fluids have entered thefractured target reservoir as a measure of extent of hydraulicfracturing.
 2. The method of claim 1, wherein spectroscopy is Ramanspectroscopy.
 3. The method of claim 2, wherein the spectroscopy isperformed by an epi-illuminated Raman spectrometer comprising a highpressure steel tube with a diameter less than 5 inches, a sapphirewindow at a bottom end of a housing to irradiate samples, a laserexcitation source, and a charge-coupled device detector.
 4. The methodof claim 1, wherein the step of determining levels of ¹²C and ¹³Cisotopes in reservoir fluids present in the fractured target reservoircomprises: removing fluid in the fractured target reservoir untilmeasured levels of solution gas and salinity for the fluid becomeconstant; and sampling the reservoir fluid.
 5. The method of claim 1,wherein the targeted reservoir is a coal bed reservoir.
 6. The method ofclaim 1, wherein a difference in respective relative levels of ¹³C inthe target reservoir before and the fractured target reservoir is usedto predict future production success or failure of the target reservoir.7. The method of claim 1, wherein the levels of ¹³C are compared to thelevels of ¹²C in order to produce a δ¹³C enrichment level.